The present disclosure relates to communication devices and particularly relates to a technique of reducing influence of noise and the like and suppressing occurrence of a communication disturbance.
A switchboard that interconnects telephone lines and forms a telephone network is generally conforming to a telephone line standard. In recent years, however, deterioration in telephone line infrastructures or applications of low-cost switchboards have caused line environments where the telephone line standard is close to the limit or the telephone line standard is un-satisfied. For example, even if a supply current from a switchboard is regulated as 20 mA (milliampere) and up, there are some switchboards incapable of supplying 20 mA and up, and other switchboards that activate protection circuits to suppress the flow of the current when the current becomes close to 20 mA. One of the causes for such environment is that data transmission and reception have been made possible by a great variety of communication means such as cell phones or mobile devices, and thereby causing neglect of maintenance of the telephone line infrastructures.
Also as for a communication device such as a facsimile device connected to a telephone line to be used is designed and controlled based on the telephone line standard, so that when a telephone network including a switchboard becomes close to the standard limit or is not satisfying the standard, there are risks of not being able to communicate normally for being sensitive to influence of on-the-line noise and the like.
With respect to the example above, if the supply current from the switchboard is below 20 mA, i.e. if a power supply capacity of the telephone line is low, a voltage level of a signal which the communication device receives becomes low, thereby becoming sensitive to the influence of on-the-line noise. A technique to suppress the occurrence of communication error caused by the influence of noise and the like has been proposed.